1. Field of the Invention
The present invention relates to apparatus for and methods of making up threaded connections for tubular goods. In another aspect, the present invention relates to apparatus for and methods of making up non-shouldered threaded connections for oilfield tubular goods. In another aspect, the present invention relates to an apparatus for and method of making up non-shouldered threaded connections manufactured according to relatively broad tolerances, to form connections having reliable pressure sealing.
2. Description of the Related Art
Threaded connections for oilfield tubular goods, for example to connect oilfield casing and tubing, can be generally characterized either as expensive or low cost connections.
The expensive connections, generally referred to as premium connections, are manufactured according to relatively stringent tolerances, and include numerous types of connections which typically have at least one "make up" shoulder, which limits the torqued rotation of the threaded male/female members. These premium connections typically also include tapered sealing surfaces which help to ensure that a reliable pressure seal between the male and female members has been obtained.
Premium connections are on the order of about 5 to about 10 times more expensive than low cost threaded connections. Thus, while premium connections are believed to perform superior to low cost connections, the lower-cost American Petroleum Institute ( "API") connections are still widely used in the oilfield, particularly in recent years during which increased emphasis has been placed on the cost of equipment used to recover oil and gas from subterranean formations.
Low cost threaded connections are manufactured according to established API specifications, and generally include an externally threaded pin and an internally threaded box or coupling.
One disadvantage with the lower cost API connections, is the lack of a shoulder to limit the torqued rotation of the threaded male/female members. Without this shoulder to limit the torqued rotation, these connections can be torqued to a distance past the proper thread engagement point, resulting in an improper sealing at the connection.
Another disadvantage with the lower cost API connections relates to the rather broad tolerances allowed by API for an externally threaded pin and an internally threaded box or coupling to comply with API thread specifications. This broad tolerance allows pins and couplings to be made up or threaded together in a manner whereby the rig hands believe that a proper pressure sealing connection has been made, when in fact the interference between the threads was insufficient to establish a fluid-tight connection once the tubular string was subject to high pressure.
The standard technique for gaging API connections is to utilize a ring and plug gage. The ring gage is screwed onto the pin (and the plug gage similarly screwed onto the box or coupling), and a measurement taken from the pin or coupling face to the gage. The allowed tolerance for the thread pitch diameter was thus expressed in inches of "standoff" from the pin or coupling face to the gage reference point. These API gages are expensive and suitable for checking only a specific sized connection.
Due to problems associated with API gaging practices, some companies have checked API threads with "proprietary" gages, which measure the actual diameter of the threads. These proprietary gages do not screw onto the threads, are not affected by the roundness of the threaded connection, and may be used to check a variety of thread sizes.
Regardless of the gaging technique utilized, API connections have a wide range of thread interference when made up to visual make-up marks on the tubular goods. A rig hand may be instructed to make up the API connections so that the box face will be axially aligned with the "last thread scratch" on the pin. This last scratch is, however, very difficult to visually detect with accuracy. Moreover, the "last scratch" does not correctly indicate the point of desired interference between the threads. If the thread is made up too far, it cannot be easily and reliably backed-out to its proper position.
Manufacturers of API buttress threads employ a make-up diamond or other marking on the pin to be used as a "bench mark" to determine if the connection is properly made up, with some operators applying their own such make-up mark. Rig hands may be instructed to make up a connection even with or a certain number of turns past this make-up diamond. While utilizing such a make-up mark is simple in concept, in actual operation difficulties are encountered. First, the mark is difficult to detect, since the mark can be easily covered by the box during the make-up operation. Also, make up of the thread may have to be halted so that pipe dope leaking from the threads onto the make-up diamond can be wiped clean for the rig hand to locate the make up mark, and this make up interruption alters the friction factors between the threads and undesirably increases galling of the threads. Finally, this method requires that a rig hand be positioned to visually sight the mark, thus requiring an additional person, and subjecting that person to a somewhat precarious situation.
Additionally, the wide tolerance range allowed by this bench mark inherently leads to a great deal of subjectivity with respect to the proper make up position.
Finally, and perhaps most important, neither the last scratch nor the bench mark technique provides a system which accurately positions the box member with respect to the end face of the pin, and the make up technique cannot be any more precise than the imprecise technique used to position the mark on the tubular goods.
The above problems have long been recognized in the industry, and accordingly many attempts have been made to improve the pressure sealing ability of API connections. Unfortunately, most of these techniques have met with limited success, in part because of they significantly increase the cost of manufacturing the connection and/or the cost and time required to perform the make up operation. One technique involves the machining of a groove in the box to accommodate a polymer seal ring. Such a groove and seal ring adds considerably to the cost of the threaded connection, and creates a need for additional parts to be shipped to the rig site where the connections are made. Also, the inclusion of the seal groove generates considerable stress in the connection, which may be detrimental to the long term reliability of the tubular string, particularly when subject to corrosive environments.
Another technique to improve the sealing ability between API connections utilizes a lubricant with a sealing material, such as Teflon bits, incorporated into the lubricant, or uses a lubricant which hardens over a period of time. Both of these techniques are designed to block the escape path of the high pressure fluid to prevent leakage past the connection. These techniques do not substantially increase the cost of the connection, but do require the rig crew to be educated to the sealing technique, and can be significantly effected by various rig environments. For example, when drilling mud is back-flowing up through the tubular as it is run into the well, the mud can prevent the proper application and/or curing of these lubricants. Also, the ability of the connection to better withstand high pressure can be improved by specifying tighter tolerance for the connection, although this approach again increases the cost of manufacturing the tubular connections and does not guarantee that the connection is going to be properly made up at the rig site.
In recent years, various techniques have been designed to measure the torque and turns on the connection to improve the sealing ability of API threads. Such a torque/turn technique may be computer monitored during the make up operation, so that the tong which controlled the make up was deactivated in response to the torque/turn system. This technique considerably increases the cost of the equipment at the well site, and generally requires the use of highly trained personnel. Moreover, the scheduling of torque/turn personnel has to be arranged with other equipment and procedures involved in running tubulars into a well, which may therefore complicate the drilling or completion operations, and thus increase operating cost. Although the torque/turn technique is widely used, it is sensitive to friction, lead, taper and shape of the thread, and to environmental factors unique to the well site, such as rig alignment and crew education.
An article in the October 1989 Petroleum Engineer International magazine discusses some of the disadvantages of using the "last scratch" technique for making up API connections. The article discloses that the actual tolerances for threads according to API gaging techniques varies more widely than had been recognized by many people in the art. The article suggests gaging each pin and box with a proprietary gage, and writing pitch diameter error on the actual pin or coupling measured. When the connection is made up, the tong operator may make a mental calculation to determine how much extra connection rotation is required based upon the mismatch between the undersized pin and the oversized box. Although the last page of the article suggests a matrix to simplify the tong operator's calculation, the proposed technique has not been widely accepted in the industry. In particular, it is believed that oilfield operators recognize that tong operators are frequently too busy with other tasks to utilize mismatch dimensions or a matrix. It is difficult for a tong operator to determine with precision the number of "extra turns" which have occurred, and the suggested technique is not sufficiently simple and reliable to be accurately and reliably used by tong operators.
U.S. Pat. No. 4,127,927 discloses a different technique for making up API threads of oilfield tubular goods. This patent recognizes the advantages of the standard and established API threads, and the API standards discussed therein are hereby incorporated by reference. This patent proposes using ring and plug gages rather than the "proprietary" gages which actually measure pitch diameter. According to the disclosure of this patent, reference lines may be applied on the outer diameter of the tubular goods using the ring and plug gages to locate these lines. Accordingly, a line would be placed on the outer diameter of the pin as a function of the gaging technique, and the position of the reference line with respect to the face end of the pin would thus vary from one pin to the next pin. The patent suggests that the coupling could be made up so that the coupling face was always at a predetermined position with respect to the reference line on the pin. Accordingly, a make up card with a single wide slot is used, and the reference line on the pin can be monitored with respect to the coupling during the make up process. The technique disclosed in this patent likewise has not been widely accepted in the industry, in part because it utilizes the API gaging technique and, therefore, does not insure that API connections are made up with the interference which will reduce or eliminate the likelihood of pressure seal loss through the threaded connection.
U.S. Pat. No. 5,040,827, discloses an improved method of making up a pressure sealing threaded connection for oilfield tubular goods, in which the actual pitch diameter of the pin thread and the box thread are measured and preferably recorded on the outer cylindrical surface of each respective member. An indicator is positioned on the pin member at a preselected location with respect to the pin face end surface. The pin member and box member are made up to form the threaded connection, while the position of the box face end surface is monitored. Make-up of the connection is terminated in response to this position and as a function of the recorded deviations on the pin and box members.
The disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus are hereinafter disclosed for reliably making up an oilfield tubular goods connection which will achieve the desired interference between the pin and box threads during the make up operation and reduce or eliminate the pressure seal loss.